Measuring the quantity of a gas in a tank

Abstract

A sensing circuit includes a passive electrical network which has a first electrical property responsive to fluid pressure, and a second electrical property responsive to fluid temperature. A network output is responsive to the first property to a first extent, and to the second property to a second extent. The first and second extents are related in the same manner as a physical property of the fluid (other than its pressure or temperature) relates to its pressure and temperature. The network can include a strain gage that has a gage factor relating the pressure in the fluid to the first electrical property, and a temperature coefficient of gage factor responsive to the fluid temperature. The network can operate according to a gas law and indicate the amount of gas in a tank.

What is claimed is: 
     
       1. A sensing circuit, comprising a passive electrical network having a first electrical property responsive to a pressure in a fluid and having a second electrical property responsive to a temperature in the fluid, the passive network having an electrical network output responsive to the first and second electrical properties to a first and second extent respectively, the first and second extents being related to each other in the same manner as a physical property of the fluid, other than the pressure in the fluid or the temperature in the fluid, relates to the pressure in the fluid and the temperature in the fluid, said physical property of the fluid varying both in response to changes in the temperature in the fluid and to changes in the pressure in the fluid. 
     
     
       2. The sensing circuit of claim 1 wherein the passive electrical network includes at least one strain gage responsive to the pressure in the fluid and wherein the electrical network output is responsive to the strain gage. 
     
     
       3. The sensing circuit of claim 2 wherein the passive electrical network further includes a thermistor responsive to the temperature in the fluid and wherein the electrical network output is responsive to the thermistor. 
     
     
       4. The sensing circuit of claim 1 wherein the passive electrical network includes a plurality of strain gages responsive to the pressure in the fluid and wherein the electrical network output is responsive to the strain gages. 
     
     
       5. The sensing circuit of claim 1 comprising a common sensing element responsive to both the pressure in the fluid and the temperature in the fluid, and having both the first and second properties, and wherein the electrical network output is responsive to the sensing element. 
     
     
       6. The sensor of claim 1 wherein the sensing circuit comprises a pressure sensing element connected in the passive electrical network and responsive to the pressure in the fluid, wherein the first electrical property is a property of the pressure sensing element, wherein the pressure sensing element has a gage factor relating the pressure in The fluid to the first electrical property, wherein the pressure sensing element has a temperature coefficient of gage factor responsive to the temperature of fluid, wherein the gage factor is the second electrical property, and wherein the passive network electrical output is responsive to the pressure sensing element. 
     
     
       7. The sensing circuit of claim 6 wherein the sensing element is a strain gage and wherein the first property is a resistance of the strain gage. 
     
     
       8. The sensing circuit of claim 7 wherein the passive electrical network further includes a thermistor responsive to the temperature in The fluid and wherein the electrical network output is responsive to the thermistor. 
     
     
       9. The sensing circuit of claim 7 further including a second strain gage connected in the passive electrical network and wherein The first and second strain gages are both mounted on the same member. 
     
     
       10. The sensing circuit of claim 6 wherein the sensing element is a semiconductor strain gage. 
     
     
       11. The sensor of claim 6 wherein the passive electrical network includes a pair of strain gages mounted on a beam, which beam is responsive to the pressure in the fluid, wherein the strain gages are located in two adjacent branches of a Wheatstone bridge, wherein the passive electrical network includes first and second resistors mounted in the remaining branches of the bridge, and wherein the passive electrical network output is responsive to the strain gages. 
     
     
       12. The circuit of claim 10 wherein the passive electrical network further includes fourth and fifth resistors in series with the first and second strain gages respectively. 
     
     
       13. A sensor, comprising: a sensor housing constructed and adapted to be mounted in a single opening in a vessel wall,   an electrical output terminal located on the housing, and   an electrical network housed in the housing and electrically connected to the electrical output terminal, the electrical network having a first electrical property responsive to a pressure in a fluid and having a second electrical property responsive to a temperature in the fluid, the electrical output terminal being responsive to the first and second electrical properties to a first and second extent respectively, the first and second extents being related to each other in the same manner as a physical property of the fluid other than the pressure in the fluid or the temperature in the fluid, relates to the pressure in the fluid and the temperature in the fluid, said physical property of the fluid varying both in response to changes in the temperature in the fluid and to changes in the pressure in the fluid, and wherein the electrical network complies a common sensing element responsive to both the pressure in the fluid and the temperature in the fluid and having both the first and second properties and wherein the output is responsive to ambient conditions at the common sensing element.   
     
     
       14. The pressure sensor of claim 13 wherein the circuitry includes a strain gage responsive to the pressure in the fluid and wherein the electrical network output is responsive to the strain gage. 
     
     
       15. A sensing circuit, comprising: passive circuit means comprising: temperature sensing means for sensing a temperature in the fluid,   pressure sensing means for sensing a pressure in the fluid, and   output terminal means responsive to the pressure sensing means, and responsive to the temperature sensing means, for providing an electrical signal proportional to a physical property of the fluid other than the temperature or pressure in the fluid according to the temperature sensing means and the pressure sensing means, said physical property of the fluid varying both in response to changes in the temperature in the fluid and to changes in the pressure in the fluid.     
     
     
       16. The sensing circuit of claim 15 wherein the pressure sensing means and the temperature sensing means comprise a common means. 
     
     
       17. A measuring method for a fluid, comprising: sensing pressure in the fluid using a sensing element,   sensing temperature in the fluid using the same sensing element, and   providing an analog voltage indicative of a value of a physical property of the fluid other than the temperature in the fluid or the pressure in the fluid, based on the step of sensing pressure in the fluid and the step of sensing temperature in the fluid, said physical property of the fluid varying both in response to changes in the temperature in the fluid and to changes in the pressure in the fluid.   
     
     
       18. The method of claim 17 wherein the step of sensing temperature in the fluid and the step of sensing pressure in the fluid employ a semiconductor strain gage. 
     
     
       19. The method of claim 17 wherein the step of sensing temperature in the fluid affects the sensitivity of the step of sensing pressure in the fluid. 
     
     
       20. The method of claim 17 wherein the step of sensing temperature in the fluid, the step of sensing pressure in the fluid, and the step of providing an analog voltage employ only passive circuit elements. 
     
     
       21. A sensing circuit, comprising a passive electrical network having a first electrical property responsive to a pressure in a fluid and having a second electrical property responsive to a temperature in the fluid, the passive network having an electrical network output responsive to the first and second electrical properties to a first and second extent respectively, the first and second extents being related to each other in the same manner as a physical property of the fluid, other than the pressure in the fluid or the temperature in the fluid, relates to the pressure in the fluid and the temperature in the fluid, and wherein the first and second extents are related according to a gas law that models real gas behavior. 
     
     
       22. The sensing circuit of claim 21 wherein the passive electrical network includes at least one strain gage responsive to the pressure in the fluid and wherein the electrical network output is responsive to the strain gage. 
     
     
       23. The sensing circuit of claim 22 wherein the passive electrical network further includes a thermistor responsive to the temperature in the fluid and wherein the electrical network output is responsive to the thermistor. 
     
     
       24. The sensing circuit of claim 21 wherein the passive electrical network includes a plurality of strain gages responsive to the pressure in the fluid and wherein the electrical network output is responsive to the strain gages. 
     
     
       25. The sensing circuit of claim 21 comprising a common sensing element responsive to both the pressure in the fluid and the temperature in the fluid, and having both the first and second properties, and wherein the electrical network output is responsive to the sensing element. 
     
     
       26. The sensor of claim 21 wherein the sensing circuit comprises a pressure sensing element connected in the passive electrical network and responsive to the pressure in the fluid, wherein the first electrical property is a property of the pressure sensing element, wherein the pressure sensing element has a temperature coefficient of gage factor relating the pressure in the fluid to the first electrical property, wherein the pressure sensing element has a temperature coefficient of gage factor responsive to ambient conditions at the temperature of the fluid, wherein the gage factor is the second electrical property, and wherein the passive network electrical output is responsive to the pressure sensing element. 
     
     
       27. The sensing circuit of claim 26 wherein the sensing element is a strain gage and wherein the first property is a resistance of the strain gage. 
     
     
       28. The sensing circuit of claim 27 wherein the passive electrical network further includes a thermistor responsive to the temperature in the fluid and wherein the electrical network output is responsive to the thermistor. 
     
     
       29. The sensing circuit of claim 27 further including a second strain gage connected in the passive electrical network and wherein the first and second strain gages are both mounted on the same member. 
     
     
       30. The sensing circuit of claim 29 wherein the first and second extents are related according to the ideal gas law. 
     
     
       31. The sensing circuit of claim 29 wherein the first and second extents are related according to the Van der Waal gas law. 
     
     
       32. The sensing circuit of claim 29 wherein the first and second extents are related according to empirical data on a particular gas. 
     
     
       33. The sensing circuit of claim 26 wherein the sensing element is a semiconductor strain gage. 
     
     
       34. The sensor of claim 26 wherein the passive electrical network includes a pair of strain gages mounted on a beam, which beam is responsive to the pressure in the fluid, wherein the strain gages are located in two adjacent branches of a Wheatstone bridge, wherein the passive electrical network includes first and second resistors mounted in the remaining branches of the bridge, and wherein the passive electrical network output is responsive to the strain gages. 
     
     
       35. The circuit of claim 34 wherein the passive electrical network further includes fourth and fifth resistors in series with The first and second strain gages respectively. 
     
     
       36. A sensor, comprising: a sensor housing constructed and adapted to be mounted in a single opening in a vessel wall,   an electrical output terminal located on the housing, and   an electrical network housed in the housing and electrically connected to the electrical output terminal, the electrical network having a first electrical property responsive to a pressure in a fluid and having a second electrical property responsive to a temperature in the fluid, the electrical output terminal being responsive to the first and second electrical properties to a first and second extent respectively, the first and second extents being related to each other in the same manner as a physical property of the fluid, other than the pressure in the fluid or the temperature in the fluid, relates to the pressure in the fluid and the temperature in the fluid, and wherein the first and second extents are related according to a gas law.   
     
     
       37. The pressure sensor of claim 36 wherein the circuitry includes a strain gage responsive to the pressure in the fluid and wherein the electrical network output is responsive to the strain gage. 
     
     
       38. The sensor of claim 36 wherein the electrical network comprises a common sensing element responsive to both the pressure in the fluid and the temperature in the fluid and having both the first and second properties and wherein the output is responsive to the common sensing element. 
     
     
       39. A sensing circuit, comprising: passive circuit means for measuring the quantity of a gas, comprising: temperature sensing means for sensing a temperature in the fluid,   pressure sensing means for sensing a pressure in the fluid, and   output terminal means responsive to the pressure sensing means, and responsive to the temperature sensing means, for providing an electrical signal proportional to a physical property of the fluid, other than the temperature or pressure in the fluid, according to the pressure sensing means and the temperature sensing means.     
     
     
       40. The sensing circuit of claim 39 wherein the pressure sensing means and the temperature sensing means comprise a common means. 
     
     
       41. A measuring method for a fluid, comprising: sensing pressure in the fluid using a sensing element,   sensing temperature in the fluid using the same sensing element, and   providing an analog voltage indicative of a value of a physical property of the fluid, other than the temperature in the fluid or the pressure in the fluid, based on the step of sensing pressure in the fluid and the step of sensing temperature in the fluid, wherein the step of providing an analog voltage proceeds according to a gas law.   
     
     
       42. The method of claim 41 wherein the step of sensing temperature in the fluid and the step of sensing pressure in the fluid employ a semiconductor strain gage. 
     
     
       43. The method of claim 41 wherein the step of sensing temperature in the fluid affects the sensitivity of the step of sensing pressure in the fluid. 
     
     
       44. The method of claim 41 wherein the step of sensing temperature in the fluid, the step of sensing pressure in the fluid, and the step of providing an analog voltage employ only passive circuit elements.